Dosage calc practice

2 tablets

1. Identify the parts: desired dose (D) = 40 mg, dose on hand (H) = 20 mg, quantity (Q) = 1 tablet.
2. Divide the desired dose by the dose on hand: 40 mg ÷ 20 mg = 2.
3. Multiply by the quantity: 2 × 1 tablet = 2 tablets.

40 mg × (1 tablet / 20 mg) = 2 tablets

Rounding: Tablets are given whole or, if scored, to the nearest half tablet; this answer is a whole number, so no rounding was needed.

Common mistake: Answering 0.5 tablet means the ratio was flipped (20 ÷ 40); the ordered dose is bigger than one tablet, so the answer must be more than one tablet.

2 tablets

1. The order (mg) and the label (mcg) use different units, so convert first: 0.1 mg × 1,000 mcg/mg = 100 mcg.
2. Divide the desired dose by the dose on hand: 100 mcg ÷ 50 mcg = 2.
3. Multiply by the quantity: 2 × 1 tablet = 2 tablets.

0.1 mg × (1,000 mcg / 1 mg) × (1 tablet / 50 mcg) = 2 tablets

Rounding: Tablets are given whole or, if scored, to the nearest half tablet; this answer is a whole number, so no rounding was needed.

Common mistake: Answering 0.002 tablet comes from dividing 0.1 by 50 without converting mg to mcg; the units must match before you divide.

0.5 tablet

1. Identify the parts: D = 12.5 mg, H = 25 mg, Q = 1 tablet.
2. Divide: 12.5 mg ÷ 25 mg = 0.5.
3. Multiply by the quantity: 0.5 × 1 tablet = 0.5 tablet (one half of a scored tablet).

12.5 mg × (1 tablet / 25 mg) = 0.5 tablet

Rounding: A half tablet is allowed only because the tablet is scored; 0.5 tablet is exact, so no rounding was needed.

Common mistake: Answering 2 tablets means the ratio was flipped (25 ÷ 12.5); the order is smaller than one tablet, so the answer must be less than one tablet.

10 mL

1. Identify the parts: D = 250 mg, H = 125 mg, Q = 5 mL.
2. Divide: 250 mg ÷ 125 mg = 2.
3. Multiply by the quantity: 2 × 5 mL = 10 mL.

250 mg × (5 mL / 125 mg) = 10 mL

Rounding: Oral liquid volumes are rounded to the tenth of a mL; 10 mL is exact, so no rounding was needed.

Common mistake: Answering 2 mL stops after the division and forgets to multiply by the 5 mL that holds each 125 mg.

7.5 mL

1. Identify the parts: D = 240 mg, H = 160 mg, Q = 5 mL.
2. Divide: 240 mg ÷ 160 mg = 1.5.
3. Multiply by the quantity: 1.5 × 5 mL = 7.5 mL.
4. Sense check: 240 mg ÷ 16 kg = 15 mg/kg, the usual single dose for a child.

240 mg × (5 mL / 160 mg) = 7.5 mL

Rounding: Oral liquid volumes are rounded to the tenth of a mL; 7.5 mL is exact, so no rounding was needed.

Common mistake: Answering 3.3 mL means the ratio was flipped (160 ÷ 240 × 5); the order is more than 160 mg, so the volume must be more than 5 mL.

10 mL

1. Convert grams to milligrams so the order matches the label: 0.5 g × 1,000 mg/g = 500 mg.
2. Divide: 500 mg ÷ 250 mg = 2.
3. Multiply by the quantity: 2 × 5 mL = 10 mL.

0.5 g × (1,000 mg / 1 g) × (5 mL / 250 mg) = 10 mL

Rounding: Oral liquid volumes are rounded to the tenth of a mL; 10 mL is exact, so no rounding was needed.

Common mistake: Answering 0.01 mL comes from using 0.5 against 250 without converting grams to milligrams; always match units before you divide.

2 mL

1. Identify the parts: D = 4 mg, H = 2 mg, Q = 1 mL.
2. Divide: 4 mg ÷ 2 mg = 2.
3. Multiply by the quantity: 2 × 1 mL = 2 mL.

4 mg × (1 mL / 2 mg) = 2 mL

Rounding: Injection volumes of 1 mL or more are rounded to the tenth of a mL; 2 mL is exact, so no rounding was needed.

Common mistake: Answering 0.5 mL means the ratio was flipped (2 ÷ 4); each mL holds only 2 mg, so 4 mg needs more than 1 mL.

0.5 mL

1. Identify the parts: D = 15 mg, H = 30 mg, Q = 1 mL.
2. Divide: 15 mg ÷ 30 mg = 0.5.
3. Multiply by the quantity: 0.5 × 1 mL = 0.5 mL.

15 mg × (1 mL / 30 mg) = 0.5 mL

Rounding: Injection volumes under 1 mL are measured to the hundredth of a mL; 0.5 mL is exact, so no rounding was needed.

Common mistake: Answering 2 mL means the ratio was flipped (30 ÷ 15); that would give 60 mg, four times the order.

0.25 mL

1. Identify the parts: D = 0.5 mg, H = 2 mg, Q = 1 mL.
2. Divide: 0.5 mg ÷ 2 mg = 0.25.
3. Multiply by the quantity: 0.25 × 1 mL = 0.25 mL.

0.5 mg × (1 mL / 2 mg) = 0.25 mL

Rounding: Injection volumes under 1 mL are measured to the hundredth of a mL with a 1 mL (tuberculin-style) syringe; 0.25 mL is exact, so no rounding was needed.

Common mistake: Rounding to 0.3 mL gives 0.6 mg, a 20% overdose of an opioid; for volumes under 1 mL keep the hundredth.

5.6 mL

1. Convert pounds to kilograms: 44 lb ÷ 2.2 lb/kg = 20 kg.
2. Find the daily dose: 45 mg/kg/day × 20 kg = 900 mg/day.
3. Every 12 hours means 2 doses a day: 900 mg ÷ 2 = 450 mg per dose.
4. Find the volume: 450 mg × 5 mL ÷ 400 mg = 5.625 mL.
5. Round to the tenth: 5.6 mL per dose.

44 lb × (1 kg / 2.2 lb) × (45 mg / kg/day) × (1 day / 2 doses) × (5 mL / 400 mg) = 5.625 mL ≈ 5.6 mL

Rounding: Weight is not rounded; the final oral volume is rounded to the tenth of a mL (5.625 mL becomes 5.6 mL).

Common mistake: Answering about 11.3 mL gives the whole daily amount (900 mg) as one dose; divide the daily dose by the number of doses first.

225 mg maximum; the 240 mg order is above the safe range

1. Convert pounds to kilograms: 33 lb ÷ 2.2 lb/kg = 15 kg.
2. Low end of the range: 10 mg/kg × 15 kg = 150 mg.
3. High end of the range: 15 mg/kg × 15 kg = 225 mg.
4. Compare: the safe range is 150 to 225 mg per dose, and the order of 240 mg is more than 225 mg.
5. The order is not safe as written: hold the dose and contact the prescriber to clarify.

33 lb × (1 kg / 2.2 lb) × (15 mg / kg) = 225 mg (maximum per dose)

Rounding: No rounding: 33 lb converts to exactly 15 kg, and the range is compared with the order as calculated.

Common mistake: Using 33 kg instead of 15 kg (skipping the lb to kg step) gives a range of 330 to 495 mg and makes the 240 mg order wrongly look too low.

0.8 mL

1. Convert pounds to kilograms: 176 lb ÷ 2.2 lb/kg = 80 kg.
2. Find the dose: 1 mg/kg × 80 kg = 80 mg.
3. Find the volume: 80 mg ÷ 100 mg/mL = 0.8 mL.

176 lb × (1 kg / 2.2 lb) × (1 mg / kg) × (1 mL / 100 mg) = 0.8 mL

Rounding: Injection volumes under 1 mL are measured to the hundredth of a mL; 0.8 mL is exact, so no rounding was needed.

Common mistake: Answering 1.76 mL uses the weight in pounds (176 mg); convert to kilograms before applying a mg/kg order.

500 min (8 hr 20 min)

1. Infusion time = volume ÷ rate: 1,000 mL ÷ 120 mL/hr = 8.333 hr.
2. Convert to minutes: 1,000 mL ÷ 120 mL/hr × 60 min/hr = 500 min.
3. Split into hours and minutes: 500 min = 8 hr (480 min) + 20 min, so 8 hr 20 min.

1,000 mL × (1 hr / 120 mL) × (60 min / 1 hr) = 500 min = 8 hr 20 min

Rounding: No rounding: the leftover 0.333… hr is converted to minutes (× 60) instead of being rounded.

Common mistake: Reading 8.33 hr as "8 hr 33 min" mixes a decimal with minutes; 0.33 hr is about 20 min (0.333 × 60).

100 mL/hr

1. Convert the time to hours: 30 min ÷ 60 min/hr = 0.5 hr.
2. Rate = volume ÷ time: 50 mL ÷ 0.5 hr = 100 mL/hr.

50 mL / 30 min × (60 min / 1 hr) = 100 mL/hr

Rounding: Pump rates are rounded to the whole mL/hr for a typical pump; 100 mL/hr is exact, so no rounding was needed.

Common mistake: Answering 1.7 mL/hr divides 50 mL by 30 minutes (that is mL per minute) and labels it per hour; pumps run in mL per hour.

167 mL/hr

1. Convert liters to milliliters: 1 L × 1,000 mL/L = 1,000 mL.
2. Rate = volume ÷ time: 1,000 mL ÷ 6 hr = 166.67 mL/hr.
3. Round to the whole number for the pump: 167 mL/hr.

1 L × (1,000 mL / 1 L) / 6 hr = 166.67 mL/hr ≈ 167 mL/hr

Rounding: This pump is set in whole mL/hr, so 166.67 mL/hr is rounded to 167 mL/hr (round only at the end).

Common mistake: Answering 0.17 mL/hr skips the liter to milliliter conversion (1 ÷ 6); the bag holds 1,000 mL, not 1 mL.

21 gtt/min

1. Convert the time to minutes: 8 hr × 60 min/hr = 480 min.
2. Total drops: 1,000 mL × 10 gtt/mL = 10,000 gtt.
3. Drops per minute: 10,000 gtt ÷ 480 min = 20.83 gtt/min.
4. Round to the whole drop: 21 gtt/min.

1,000 mL / 480 min × 10 gtt/mL = 20.83 gtt/min ≈ 21 gtt/min

Rounding: Drops cannot be split, so gtt/min is rounded to the nearest whole drop (20.83 becomes 21).

Common mistake: Answering 1,250 gtt/min divides by 8 hours instead of 480 minutes; a gravity drip is counted per minute.

31 gtt/min

1. Convert the time to minutes: 4 hr × 60 min/hr = 240 min.
2. Total drops: 500 mL × 15 gtt/mL = 7,500 gtt.
3. Drops per minute: 7,500 gtt ÷ 240 min = 31.25 gtt/min.
4. Round to the whole drop: 31 gtt/min.

500 mL / 240 min × 15 gtt/mL = 31.25 gtt/min ≈ 31 gtt/min

Rounding: gtt/min is rounded to the nearest whole drop (31.25 becomes 31).

Common mistake: Answering 125 gtt/min uses the mL/hr rate (500 ÷ 4) as if it were drops per minute; convert hours to minutes and multiply by the drip factor.

75 gtt/min

1. Convert the time to minutes: 4 hr × 60 min/hr = 240 min.
2. Total drops: 300 mL × 60 gtt/mL = 18,000 gtt.
3. Drops per minute: 18,000 gtt ÷ 240 min = 75 gtt/min.
4. Shortcut check: with 60 gtt/mL tubing, gtt/min equals mL/hr, and 300 mL ÷ 4 hr = 75 mL/hr.

300 mL / 240 min × 60 gtt/mL = 75 gtt/min

Rounding: gtt/min is rounded to the nearest whole drop; 75 is exact, so no rounding was needed.

Common mistake: Answering 19 gtt/min uses a 15 gtt/mL macrodrip factor instead of the 60 gtt/mL printed on the microdrip tubing.

1.5 mL

1. Use the concentration printed on the label after mixing: 330 mg/mL (the powder adds volume, so it is not 1,000 mg in 2.5 mL).
2. Find the volume: 500 mg ÷ 330 mg/mL = 1.52 mL.
3. Round to the tenth: 1.5 mL.

500 mg × (1 mL / 330 mg) = 1.52 mL ≈ 1.5 mL

Rounding: Injection volumes of 1 mL or more are rounded to the tenth of a mL (1.52 mL becomes 1.5 mL).

Common mistake: Answering 1.3 mL assumes 1,000 mg in 2.5 mL (400 mg/mL); the powder displaces volume, so use the label's 330 mg/mL.

1.4 mL

1. Use the label concentration after mixing: 350 mg/mL.
2. Find the volume: 500 mg ÷ 350 mg/mL = 1.43 mL.
3. Round to the tenth: 1.4 mL.

500 mg × (1 mL / 350 mg) = 1.43 mL ≈ 1.4 mL

Rounding: Injection volumes of 1 mL or more are rounded to the tenth of a mL (1.43 mL becomes 1.4 mL).

Common mistake: Answering 0.7 mL means the ratio was flipped (350 ÷ 500); 500 mg is more than one mL's worth of 350 mg.

15 mL

1. Check the label concentration: 1 g = 1,000 mg, and 1,000 mg ÷ 20 mL = 50 mg/mL.
2. Find the volume: 750 mg ÷ 50 mg/mL = 15 mL.

750 mg × (1 mL / 50 mg) = 15 mL

Rounding: Volumes of 1 mL or more are rounded to the tenth of a mL; 15 mL is exact, so no rounding was needed.

Common mistake: Answering 20 mL adds the whole vial (1 g), which is 250 mg more than the 750 mg ordered.

1,200 units/hr

1. Find the concentration: 25,000 units ÷ 250 mL = 100 units/mL.
2. Multiply by the rate: 12 mL/hr × 100 units/mL = 1,200 units/hr.

12 mL/hr × (25,000 units / 250 mL) = 1,200 units/hr

Rounding: No rounding was needed; the result is a whole number of units per hour.

Common mistake: Answering 0.12 units/hr divides the rate by the concentration (12 ÷ 100); mL/hr times units/mL gives units/hr.

28.8 mL/hr

1. Find the hourly dose: 18 units/kg/hr × 80 kg = 1,440 units/hr.
2. Find the concentration: 25,000 units ÷ 500 mL = 50 units/mL.
3. Find the rate: 1,440 units/hr ÷ 50 units/mL = 28.8 mL/hr.

18 units/kg/hr × 80 kg × (500 mL / 25,000 units) = 28.8 mL/hr

Rounding: This pump accepts tenths, so the rate is kept to the tenth of a mL/hr; 28.8 mL/hr is exact.

Common mistake: Answering 14.4 mL/hr uses 100 units/mL, the concentration of a 250 mL bag; always check the bag volume on the label.

7 mL/hr

1. Find the hourly dose: 0.1 units/kg/hr × 70 kg = 7 units/hr.
2. Find the concentration: 100 units ÷ 100 mL = 1 unit/mL.
3. Find the rate: 7 units/hr ÷ 1 unit/mL = 7 mL/hr.

0.1 units/kg/hr × 70 kg × (100 mL / 100 units) = 7 mL/hr

Rounding: No rounding was needed; 7 mL/hr is a whole number.

Common mistake: Answering 70 mL/hr drops the decimal in 0.1 units/kg/hr, a tenfold overdose; insulin is a high-alert drug, so have a second nurse double-check.